The invention relates to a method for purifying gases from internal combustion engines and a apparatus for carrying out the method.
A known method, and an apparatus for carrying out the method, for purifying exhaust gases using a catalyzer system is described in Japanese Patent document JP 4-287 820 (A). This system includes a first catalyzer (in the exhaust gas flow direction) and at least one second catalyzer, arranged downstream of the first catalyzer, for the conversion of HC, CO and NO.sub.x exhaust gas constituents. Exhaust gases flow through both catalyzers during the entire operating period of the engine. The first catalyzer is at least suitable for the catalytic conversion of HC and CO exhaust gas constituents. In a first step of the known method to reduce the HC and CO emissions, the first catalyzer is operated over-stoichiometrically during the cold starting phase.
A method for purifying exhaust gases from internal combustion engines having a catalyzer system is likewise known from European Patent document EP 0 427 494 A2. This system includes a first catalyzer (in the exhaust gas flow direction) and a second catalyzer arranged downstream of the first catalyzer. The first catalyzer, which is configured as a starting catalyzer, is near the engine and has a lower starting temperature. The second catalyzer, which is configured as an under-body catalyzer, is remote from the engine. Both catalyzers have exhaust gases flowing through them during the entire operating period of the engine. Each of the two catalyzers is a conventional three-way catalyzer.
It is known from German Patent document DE 42 05 496 C1 to bypass the catalyzer near the engine, in order to protect against thermal overload when the starting temperature of a catalyzer which is remote from the engine is reached. However, this system is of a different generic type.
As general background, reference is also made to the publications DE 22 08 190 C3, DE 25 54 359 C2, DE 33 37 903 A1, DE 36 42 018 A1, EP 0 387 611 A2, EP 0 410 440 A1 and U.S Pat. No. 5,051,244.
A disadvantage of the known methods in the case of catalyzers, such as starting catalyzers, arranged near the engine, for example, is that very high thermal loading of the catalyzer occurs, considered over the entire operating period of the engine. This results in accelerated ageing of the catalyzer. The thermal loading is partly due to the relatively high heat quantity associated with the exhaust gas in the region near the engine. A further substantial contribution to the thermal loading, particularly in the case of a warm engine, is provided by the exothermal conversion of hydrocarbon (HC) and carbon monoxide (CO) exhaust gas constituents in conventional catalyzers near the engine. This conversion causes an average increase in temperature relative to the catalyzer inlet on the order of around 100.degree. C. In small local catalyzer regions, furthermore, the increase in temperature is substantially larger so that this additional increase in temperature makes a decisive contribution to the accelerated ageing of the catalyzer near the engine in cases where the associated exhaust gas has a high exhaust gas temperature and a large quantity of heat.
A further disadvantage of known catalyzer systems, where the starting catalyzer near the engine can be optionally by-passed in order to protect the starting catalyzer from thermal overload in the warm engine condition, is the relatively complicated construction and expensive manufacture of such catalyzer systems, as well as the by-pass conduits and control devices.
The present invention is based on need for designing a method of the generic type, and an apparatus for carrying out the method, for purifying exhaust gases from internal combustion engines in a manner which is as simple and low-cost as possible and in such a way that a substantially better endurance of the catalyzer system is achieved for the same good reduction in the exhaust gas pollutants.
These needs are met according to the present invention by providing a method for purifying exhaust gases from internal combustion engines having a catalyzer system which includes a first catalyzer (in the exhaust gas flow direction) and at least one second catalyzer, arranged downstream of the first catalyzer, for the conversion of HC, CO and NO.sub.x exhaust gas constituents. The exhaust gases flow through both catalyzers during the entire operating period of the engine. The first catalyzer is suitable for at least the catalytic conversion of HC and CO exhaust gas constituents. In a first step of the method, the first catalyzer is operated over-stoichiometrically during the cold starting phase in order to reduce the HC and CO emissions. In a further step of the method with a .lambda.-controlled stoichiometric exhaust gas composition, the catalytic conversion of the exhaust gas in the first catalyzer is at least de-activated and takes place in the second catalyzer.
One advantage of the method according to the present invention is the fact that, despite the permanent flow of exhaust gas through the first catalyzer (first in the exhaust gas flow direction), this catalyzer is subjected, during the entire engine operation, to a temperature which is, at a maximum, slightly above the exhaust gas temperature at the catalyzer inlet in the warm engine condition. As a consequence, the thermal loading of the first catalyzer (first in the exhaust gas flow direction) is considerably reduced and its endurance is therefore substantially improved. The higher endurance is advantageously achieved and in accordance with the invention in that the first catalyzer is only catalytically active in the cold starting phase with an over-stoichiometric exhaust gas composition, in a manner described in more detail below, and is largely catalytically inactive in the case of .lambda.-controlled, stoichiometric exhaust gas composition (from attainment of operating temperature of the internal combustion engine).
A further advantage of the method according to the present invention is provided by a first catalyzer which is designed as an oxidation catalyzer and is deactivated according to the invention in the case of a controlled exhaust gas flow. This is because such a catalyzer also secures the three-way function of downstream three-way catalyzer in that the (de-activated) oxidation catalyzer lets the HC and CO pass chemically unaltered in the case of a .lambda.-controlled exhaust gas flow and, by this means, makes these compounds available as auxiliary reaction partners in the three-way catalyzer.
A further advantage of the method according to the invention may be seen in the fact that the possibility of avoiding by-passing in the case of starting catalyzers near the engine makes structural simplification attainable and makes a substantial reduction in the manufacturing costs of the complete catalyzer system achievable.
By means of the apparatus, according to the invention for carrying out the method, and due to the prevention of or very great reduction in the alternate adsorption and desorption of oxygen in the first catalyzer (first in the exhaust gas flow direction), an exothermic conversion of HC and CO exhaust gas constituents is prevented or is very greatly reduced in the case of a .lambda.-controlled stoichiometric exhaust gas composition even above the starting temperature of the first catalyzer. The exothermic conversion of HC and CO is prevented because compensation does not now occur by adsorption and desorption of oxygen for the pulsating concentrations of the exhaust gas components caused by the .lambda.-control of the engine (rich and lean phases of the exhaust gas in the immediate vicinity of .lambda.=1), as is necessary in the case of conventional catalyzers in order to maintain its "three-way effect" as for an exact .lambda.=1. It is possible to dispense with the alternative adsorption and desorption of oxygen in the first catalyzer because, in the cold starting phase, this catalyzer operates without .lambda.-control and with an excess of air and, therefore, sufficient oxygen for the oxidation of HC and CO is provided in the exhaust gas itself. In addition, a reduction of NO.sub.x during the cold starting phase is unnecessary because the quantity of it in the exhaust gas is negligibly small.
A particularly advantageous structural configuration of the apparatus according to the present invention for carrying cut the method, and one which is preferentially suitable for strict emission requirements, is achieved by arranging the three catalyzers in series. This is due, on the one hand, to the reduction described above of the cold starting emissions by the starting catalyzer (three-way catalyzer or oxidation catalyzer) arranged near the engine and, on the other hand, by a very effective conversion of all three exhaust gas components NO.sub.x, CO and HC in the warm condition of the engine. As already mentioned above, HC and CO are available in sufficient quantity, in the case of the warm engine, as auxiliary reaction partners for the catalytic reduction of NO.sub.x in the downstream second catalyzer because the first catalyzer has been de-activated. The second catalyzer is a three-way catalyzer optimized for the reduction of NO.sub.x. The remaining HC and CO residues after the second catalyzer are converted in the oxidation catalyzer arranged downstream of the second catalyzer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.